The present invention relates to a device for the control of a double-acting hydraulic jack, of the type comprising, in a main circuit, a distributor assembly controlled by electrical means and commanding the putting of each of the two chambers of the jack selectively in relation with a source of fluid under high pressure or with a reservoir of fluid at low pressure, and an auxiliary circuit controlled by means sensitive to the position of the piston of the jack.
In many uses, it is desirable, in the event of a failure, that the piston of the jack should return automatically to the rest position, for example to the mid-position where a double-acting jack is concerned. U. S. Pat. No. 4,953,445 makes known a device for the control of a double-acting hydraulic jack, in which, according to one embodiment, a failure in the hydraulic circuit under high pressure causes the piston of the jack to be blocked in the position which it occupied before the failure, an arrangement being provided in the event of a failure of the electrical control circuit only according to another embodiment. This letter embodiment makes it necessary to mount a device for detecting the position of the piston of the jack and therefore to equip the jack with two plugs of complex form, through which the output rod of the jack passes and which are equipped with distributors. The slides of these distributors must be machined with very high accuracy, typically of the order of a micron, and moreover must be paired very exactly in order to ensure that the device has perfect symmetry. In fact, in the event of a failure, each distributor must be capable of ensuring the admission of fluid under pressure into one of the chambers of the 15 jack, while the other distributor must make it possible to discharge exactly the same volume of fluid contained in the other chamber toward a low-pressure reservoir, so that the return movement of the jack piston is obtained with complete reliability.